Section 10.2: Theoretical Acid–Base Definitions

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Section 10.2: Theoretical Acid–Base Definitions
Mini Investigation: pH and Dilutions, page 474
A. Answers may vary. The factor is ten times as each sample is diluted.
B. Answers may vary. Each pH value continually increased.
0.1 M pH = 1.95
0.01 M pH = 2.81
0.001 M pH = 3.50
0.0001 M pH = 4.26
C. Answers may vary. From the values above, a pH of 5.5 could be a fair prediction.
Section 10.2 Questions, page 475
1. (a) HF(aq) is an Arrhenius acid because it dissociates in water to produce hydrogen ions.
(b) Ba(OH)2(aq) is an Arrhenius base because it dissociates into cations and hydroxide ions in
water.
(c) H2SO4(aq) is an Arrhenius acid because it dissociates in water to produce hydrogen ions.
(d) NH4OH(aq) is an Arrhenius base because it dissociates into cations and hydroxide ions in
water.
2. Theories are often described as being “valid” rather than “correct” because they provide a
plausible explanation for a naturally occurring phenomenon. However, there may be limitations
to a theory or there may be situations in which a theory is invalid. This explains why theories are
not usually described as being “correct.” For example, Lavoisier believed acids contained
oxygen because he observed that sulfur and phosphorus burned in air to produce acidic solutions.
However, the discovery of acidic substances that did not contain oxygen proved his theory to be
invalid.
3. (a) Diagram (iii) represents hydrochloric acid because no molecules are present. Since
hydrochloric acid is a strong acid, it completely ionizes into hydrogen and chloride ions in water.
(b) Diagram (ii) is ethanoic acid because it shows some of the molecules have ionized. Since
ethanoic acid is a weak acid, only a fraction of its molecules ionize.
(c) Diagram (i) is hydrogen chloride gas because there is no evidence of ionization. Only
molecules are seen.
4. An acid is considered to be a weak acid if it only partially ionizes in water. However, a large
quantity of the acid may be dissolved in water, which would make the acid concentrated. Thus, it
is possible for the acid to be both weak and concentrated.
5. The pH decreases as a solution becomes more acidic. However, a strong acid may have the
same pH as a weak acid if the strong acid is diluted.
6. (a) Step 1. Write the formula equation for the reaction.
HBr(aq) + KOH(aq) → KBr(aq) + H2O(l)
Step 2. Write the total ionic equation
H+(aq) + Br–(aq) + K+(aq) + OH–(aq) → K+(aq) + Br–(aq) + H2O(l)
Step 3. Cancel the spectator ions and write the net ionic equation.
H+(aq) + OH–(aq) → H2O(l)
(b) Step 1. Write the formula equation for the reaction.
2 HNO3(aq) + Ba(OH)2(aq) → KBr(aq) + H2O(l)
Step 2. Write the total ionic equation
2 H+(aq) + 2 NO3–(aq) + Ba2+(aq) + 2 OH–(aq) → 2 NO3–(aq) + Ba2+(aq) + 2 H2O(l)
Copyright © 2011 Nelson Education Ltd.
Chapter 10: Acids and Bases
10.2-1
Step 3. Cancel the spectator ions and write the net ionic equation.
H+(aq) + OH–(aq) → H2O(l)
7. The strong acid has a lower pH because strong acids ionize completely in water while weak
acids only partially ionize in water.
8. (a) Stomach acid is 100 times more acidic than orange juice.
(b) The acidity of orange juice is not a health risk because the digestive juices in the stomach are
much more acidic than orange juice.
9. Safety precautions that you should use when working with strong acids and bases include
wearing eye protection, gloves, and an apron. Adequate ventilation may also be required.
10. (a) Lemon juice is 103 times (pH 5 – pH 2) more acidic than black coffee.
(b) Black coffee is 105 times (pH 10 – pH 5) more acidic than milk of magnesia.
(c) Battery acid is 107 times (pH 7 – pH 0) more acidic than pure water at 25 °C.
(d) Tomatoes are 107 times (pH 11 – pH 4) more acidic than aqueous ammonia.
11. (a) Aqueous ammonia is 104 times (pH 11 – pH 7) more basic than pure water.
(b) 1 mol/l NaOH(aq) is 104 times (pH 14 – pH 10) more basic than milk of magnesia.
(c) Tomatoes are 102 times (pH 4 – pH 2) more basic than lemon juice.
(d) Milk of magnesia is 105 times (pH 10 – pH 5) more basic than black coffee.
12. (a) According to Arrhenius, both sulfuric acid solution and barium hydroxide solution
conduct electricity well because they contain an ample supply of ions.
(b) H2SO4(aq) + Ba(OH)2 → BaSO4(s) + 2 H2O(l)
(c) The addition of barium hydroxide to sulfuric acid results in a reduction in electrical
conductivity. The salt produced during this neutralization is barium sulfate, which has a very low
solubility. Consequently, too few barium and sulfate ions are present in solution for conductivity
to occur. If sodium hydroxide had been used to neutralize sulfuric acid, the resulting solution
would be a good conductor. The salt produced when sulfuric acid is neutralized with sodium
hydroxide is sodium sulfate, which is a highly soluble compound. The resulting solution is a
good conductor because of the high concentration of sodium and sulfate ions present.
13. (a) After Arrhenius presented his theory, he initially had difficulty finding a teaching position
at a university in his native Sweden. Instead, he had to settle for a position at a technical high
school. During this period, he received a travel grant from the Swedish Academy of Sciences,
which allowed him to work abroad. This allowed him to spread the word about his theory. Over
time, Arrhenius's theory was gradually accepted.
(b) The scientific community finally recognized Arrhenius for his contributions to chemistry by
awarding him a Nobel Prize in Chemistry in 1903 for his theory on the dissociation of
electrolytes.
Copyright © 2011 Nelson Education Ltd.
Chapter 10: Acids and Bases
10.2-2
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